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What are Small Strongyles?

An invasion of small strongyles is so prevalent in horses, that virtually 100% are infected with some species of the parasite. This is due to many factors, such as the fact that horses are constantly exposed to them in their feeding pastures, and that dewormers are not completely effective against parasitic infections. Once cyathostomins are ingested, they invade the linings of the cecum and colon in the large intestine. Here they molt and grow inside fibrous cysts for weeks to years. Once the cysts erupt and release the cyathostomin larvae, they invoke an inflammatory response in the surrounding tissues. The adults then live near the mucosa and feed on organic material ingested by the horse, releasing eggs that get passed in the feces.

Small strongyles are parasitic worms that live in the large intestine of a horse. They are one type of over a hundred species of internal parasites that can invade horses, and are also called trichonemes, cyathostomes, and cyathostomins. Symptoms of weight loss and diarrhea are indicative of inflammation and hemorrhage in the cecum and colon due to the release of chemicals by the cyathostomins inside the affected horse’s body.

There are over forty species of small strongyles, ten of which are the primary invaders in horses. These parasites go through five stages of life.

Stage 1

The egg embryonates in the environment to this first stage larvae which feeds on organic material, so long as the temperature and conditions are right.

Stage 2

The egg hatches as the larvae develop to the second stage, also feeding on organic material.

Stage 3

This is the infective stage, as third stage larvae crawl up on pasture forage and are ingested by grazing horses. Since the larvae are almost microscopic in size, one droplet of water can house hundreds of larvae. Once inside the horse, the larvae form cysts inside the large intestinal wall and develop for several weeks to years, subsisting on stored energy reserves. They may progress to the fourth stage after a period of dormancy, remaining completely protected inside the cysts.

Stage 4

Stage four larvae erupt from the cysts in the intestinal wall, producing chemicals which cause an inflammatory response resulting in clinical signs. The larvae feed on the mucosa and can rupture capillaries, causing bleeding. When massive numbers of larvae emerge at once, it can result in larval cyathostomiasis, or an excessive protein and fluid loss that can lead to death of the horse.

Stage 5

This is the adult stage, where the parasites molt into fully grown worms that attach to the gut and feed on organic material digested by the horse. The adults produce eggs, which are passed through the feces of the horse. Under the right environmental conditions, those eggs can hatch in the environment, and progress to be able to infect a new host.

Causes of Small Strongyles in Horses

The cause of small strongyles in horses is the ingestion of the cyathostomin larvae. The larvae then grow inside the horse. The main avenues of transmission are:

Pasture forage

Moist forage

Roughs that are grazed

Pasture that is dragged or harrowed

Diagnosis of Small Strongyles in Horses

The symptoms of small strongyles are similar to many other conditions, and your veterinarian will need to know any incidence of parasites in your horse, or others in the population. After a physical exam, a diagnosis of small strongyles can still be challenging, as the eggs are not seen in fecal samples. However, later stage larvae are bright red, and can be seen in the feces. A laparotomy can be performed to obtain a biopsy of the large intestine for testing, and a rectal biopsy can also be taken. After the death of a horse, cyathostomin larvae can be detected in the mucosa in the intestine, seen as small, gray dots with a gritty texture.

Treatment of Small Strongyles in Horses

Treatment of small strongyles aims for two goals. The first is to minimize the amount of parasites inside of your horse, or horses. The other is to minimize the level of exposure to all the horses in your population by reducing the number of infective stage larvae.

Individual horses are treated with appropriate doses of anthelmintics, or antiparasitic drugs, based on body weight. Some strains of small strongyles are resistant to certain drugs. Drug rotation may be employed to find the right class of medication needed to be effective against the cyathostomin parasite. Egg counts are used to measure the success of the drug, and can be taken from feces at the time of treatment, and then compared to those taken 10 to 14 days later. Later stage larvae may be harder to treat with anthelmintics. If your horse does not respond to treatments, corticosteroids and other supportive care may be recommended.

Next, the population must be treated to help prevent the spread of the parasites through lowering the level of egg deposition on pastures and forage, as well as to manage parasite levels in individuals. Such techniques employed are herd deworming at predetermined intervals, fecal egg counts taken on a regular basis to determine the efficacy of deworming products, and pasture rotation. The fecal egg count reduction testing (FECRT) and McMaster technique are both popular methods of testing drug resistance. There is a risk that frequent deworming of your herd can cause drug resistance in the parasites, so discuss with your veterinarian the best options for your particular farm. Herbicides are also sometimes employed in pastures.

Recovery of Small Strongyles in Horses

Your horse will need monitoring to assure that the parasites are responding to treatment. By using egg counts, you can determine if the drug chosen is effective, and are then able to switch drug classes if the current treatment is not working. Some adult horses can develop an immunity to reinfection of the parasites. Selective treatment of only infected horses should be discussed with your veterinarian. Other ways to manage cyathostomin levels on your farm include:

Removing feces from pastures and paddocks

Regularly treating all horses in the population at the same time

Quarantining and deworming boarded, new, or returning horses

Anthelmintic rotation

Monitoring egg levels in fecal samples periodically

Pastures should be dragged or harrowed during the hottest months and left vacant for many weeks afterward

Harrowing pastures in cooler regions at the end of grazing seasons to reduce the survival of infective larvae over the winter